1. Technical Field
The present invention relates to a laminated high strength monolithic carbon foam material that is especially useful for the production of decking for naval vessels and the like. More particularly, the present invention relates to reinforced carbon foam sandwiched between two composite facesheets, and exhibiting improved strength, weight and density characteristics desired for decking applications. The invention also includes methods for the production of such sandwiches.
2. Background Art
Decking for naval vessels and the like presents unique challenges. While metals such as steel and aluminum have traditionally been used where high strength is desired, the weight of these materials is such that the configuration of the vessels has to be specifically designed with the weight of the decking material in mind. In addition, metallic decking transmits heat and vibration, requiring specialized dampeners and other materials to be employed, at significant cost, and added weight. While composite laminates of for example, foams and wood products, have been suggested for decking applications, a composite having the required strength to weight ratio has not yet been found feasible. Added to this is the fact that laminate sandwiches developed to date are prone to failure through shear stresses—the tendency of the layers to fail laterally along the major surfaces of the sandwich.
Carbon foams have attracted considerable recent activity because of their properties of low density, coupled with either very high or low thermal conductivity. Conventionally, Carbon foams are prepared by two general routes. Highly graphitizable foams have been produced by thermal treatment of mesophase pitches under high pressure. These foams tend to have high thermal and electrical conductivities. For example, in Klett, U.S. Pat. No. 6,033,506, mesophase pitch is heated while subjected to a pressure of 1000 psi to produce an open-cell foam containing interconnected pores with a size range of 90-200 microns. According to.
Klett, after heat treatment to 2800° C., the solid portion of the foam develops into a highly crystalline graphitic structure with an interlayer spacing of 0.366 nm. The foam is asserted to have compressive strengths greater than previous foams (3.4 MPa or 500 psi for a density of 0.53 gm/cc).
In Hardcastle et al. (U.S. Pat. No. 6,776,936) carbon foams with densities ranging from 0.678-1.5 gm/cc are produced by heating pitch in a mold at pressures up to 800 psi. The foam is alleged to be highly graphitizable and provide high thermal conductivity (250 W/m° K).
According to H. J. Anderson et al. in Proceedings of the 43d International SAMPE Meeting, p 756 (1998), carbon foam is produced from mesophase pitch followed by oxidative thermosetting and carbonization to 900° C. The foam has an open cell structure of interconnected pores with varying shapes and with pore diameters ranging from 39 to greater than 480 microns.
Rogers et al., in Proceedings of the 45th SAMPE Conference, pg 293 (2000), describe the preparation of carbon foams from coal-based precursors by heat treatment under high pressure to give materials with densities of 0.35-0.45 g/cc with compressive strengths of 2000-3000 psi (thus a strength/density ratio of about 6000 psi/g/cc). These foams have an open-celled structure of interconnected pores with pore sizes ranging up to 1000 microns. Unlike the mesophase pitch foams described above, they are not highly graphitizable. In a recent publication, the properties of this type of foam were described (High Performance Composites September 2004, pg. 25). The foam has a compressive strength of 800 psi at a density of 0.27 g/cc or a strength to density ratio of 3000 psi/g/cc.
Stiller et al. (U.S. Pat. No. 5,888,469) describes production of carbon foam by pressure heat treatment of a hydrotreated coal extract. These materials are claimed to have high compressive strengths of 600 psi for densities of 0.2-0.4 gm/cc (strength/density ratio of from 1500-3000 psi/g/cc). It is suggested that these foams are stronger than those having a glassy carbon or vitreous nature which are not graphitizable.
Carbon foams can also be produced by direct carbonization of polymers or polymer precursor blends. Mitchell, in U.S. Pat. No. 3,302,999, discusses preparing carbon foams by heating a polyurethane polymer foam at 200-255° C. in air followed by carbonization in an inert atmosphere at 900° C. These foams have densities of 0.085-0.387 g/cc and compressive strengths of 130 to 2040 psi (ratio of strength/density of 1529-5271 psi/g/cc).
In U.S. Pat. No. 5,945,084, Droege described the preparation of open-celled carbon foams by heat treating organic gels derived from hydroxylated benzenes and aldehydes (phenolic resin precursors). The foams have densities of 0.3-0.9 g/cc and are composed of small mesopores with a size range of 2 to 50 nm.
Mercuri et al. (Proceedings of the 9th Carbon Conference, pg. 206 (1969) prepared carbon foams by pyrolysis of phenolic resins. For foams with a density range of 0.1-0.4 gm/cc, the compressive strength to density ratios were from 2380-6611 psi/g/cc. The pores were ellipsoidal in shape with pore diameters of 25-75 microns) for a carbon foam with a density of 0.25 gm/cc.
Stankiewicz (U.S. Pat. No. 6,103,149) prepares carbon foams with a controlled aspect ratio of 0.6-1.2. The patentee points out that users often require a completely isotropic foam for superior properties with an aspect ratio of 1.0 being ideal. An open-celled carbon foam is produced by impregnation of a polyurethane foam with a carbonizing resin followed by thermal curing and carbonization. The pore aspect ratio of the original polyurethane foam is thus changed from 1.3-1.4 to 0.6-1.2.
Unfortunately, carbon foams produced by the prior art processes are not effective for many high strength applications, such as naval decking, where high strength must be balanced with the need for light weight. In other words, decking, especially for an application such as a naval vessel, needs to be strong enough to withstand the weight to be applied to the decking, whether it is due to people aboard ship or equipment installed on the decking. At the same time, the decking must be lighter weight than steel or aluminum, the materials being replaced in such applications, in order to be considered worthwhile. The foams generally available are not monolithic and do not have the strength and strength to density requirements for such application. In addition, open-celled foams with highly interconnected pores have porosities making them ill-placed for such applications.
In U.S. Pat. No. 6,291,049, a foam core laminate is disclosed, having a core with opposing top and bottom surfaces; a plurality of discrete pins disposed through the core and extending beyond the top and bottom surfaces of the core; a face sheet on the top core surface; and a face sheet on the bottom core surface, the ends of each pin bent over and lying between the respective core surfaces and the facesheets. However, although useful for many applications, the resulting sandwich does not have the strength and weight characteristics needed for naval decking.
What is desired, therefore, is a laminate sandwich containing a carbon foam, where the cell structure, strength, strength to density ratio and resistance to shear stresses make the laminate suitable for use as decking for naval vessels as well as in other applications. Indeed, a combination of characteristics, including strength to density ratios and resistance to shear stress higher than contemplated in the prior art, have been found to be necessary for use of a carbon foam in decking applications. Also desired is a process for preparing such laminate sandwiches.